A liquid crystal display screen generally appears in the form shown on FIG. 1. The actual screen ECR is constituted by addressing lines L and addressing columns C, a matrix of pixels P, each connected to a transistor TFT whose state is controlled by the associated line L and column C.
This screen is controlled by a line control circuit CCL which sequentially applies to the lines an addressing voltage (for example, several tens of volts) and by a column control circuit CCC which applies to all the columns voltages reflecting the light intensity of the points to be displayed on the addressed line. The overall image is thus displayed line by line.
The column control circuit CCC receives a video signal SV delivered by a video circuit CV. Generally speaking, this signal is made up of three components corresponding to the three primary components of a color image.
If the ECR screen has 162 columns, the circuit CCC includes 162 parallel-disposed elementary column control circuits and 162 outputs connected to the various columns. Each elementary column control circuit (still technically known as a "column driver") includes a sample holder whose function is to sample the video signal at a specific moment and corresponding to the column to be controlled and to retain this sample on the column throughout the period for addressing a line (known as a "sample-and-hold" function).
So as to verify the proper functioning of such a column control circuit, at the moment the latter is produced, voltages are measured with the aid of points placed in contact with various points of the integrated circuit.
This conventional technique of carrying out tests under points does have the drawback of being that much more difficult to implement when the number of points to be tested is large.